The high-speed power trains of performance cars and aircraft often require the use of precision bevel and/or hypoid gears. Such precision gears are presently manufactured by a long and careful process which begins with the manufacture of a roughed gear workpiece, the teeth of this roughed gear having flanks which are only a few thousandths of an inch (less than 0.1 mm) larger than its desired dimensions when finished. This roughed workpiece is heat-treated to harden the surfaces of the teeth, and then it is finished in the following manner: (a) a finishing machine is set up to grind the roughed teeth to their final shape; (b) prior to being mounted on the finishing machine, each roughed workpiece is checked for serious nicks, burrs, or extreme dimensional errors that might prevent proper finishing; (c) the first roughed workpiece of each heat-treated lot is mounted on the finishing machine and finished; (d) this first gear is then removed from the finishing machine and sent to a test machine where its accuracy is carefully checked; (e) the setup of the finishing machine is then modified in accordance to the findings of the test; (f) the tested first gear is then returned to the machine and refinished using the corrected settings; and (g) the refinished first part is removed and retested. In many instances, steps (d) through (g) may have to be repeated several times before machine settings produce an acceptably-shaped gear. When the shape of the test part is deemed acceptable, the remaining gears in each heat-treated lot are then ground, each being given pre-machining and post-machining checks. Further, during the processing of the remaining gears in the lot, the post-machining dimensions of the finished gears is monitored and the setup of the machine is modified as necessary to adjust for any accuracy variations due to thermal changes, etc.
Of course, each time a workpiece is mounted or remounted on the machine tool, it must be stock divided (i.e., the grinding or cutting tool must be appropriately positioned relative to the pre-cut teeth on the workpiece). Such stock division is usually done by an operator when precision gears are being finished. There are also known automatic stock division systems. Some of these known systems use non-contact probes to sense the position of the flanks of the teeth of the gear-shaped workpiece. However, since these non-contact systems are not considered accurate enough for precision gear finishing operations, contact-type probes are often used instead for automatic stock division. Accurate stock division requires that the flanks of several teeth be measured, and such multiple measurements by a contact probe is quite time-consuming.
During the finishing process, the grinding wheel (which is used to shape the teeth of the workpiece) must be dressed at regular intervals to assure its accuracy and to maintain an appropriately sharp grinding surface. However, each time the wheel is dressed, its size and shape is altered, and so the machine's tool and work supports must be carefully reset after each such dressing operation to assure that the grinding wheel is accurately positioned relative to the workpiece before further grinding operations are initiated.
It can be appreciated that this multiple handling and testing takes considerable time and requires expert machine operators, and therefore, that each precision gear is a relatively expensive product.
The machines presently used for finishing bevel and hypoid gears are quite complex, the grinding wheel or cutting tool being mounted in a spindle which itself is moved eccentrically in a rotating cradle journaled in the tool support. In addition, the tool spindle is often mounted in a further mechanism which tilts the spindle relative to its support to adjust the angular position of the tool axis with respect to the axis of the cradle. Such conventional bevel and hypoid gear generating machines require nine or more machine settings (also known as "setup axes") for appropriately positioning the tool with respect to the gear-shaped work workpiece, and the general orientation of the tool and work heads of these traditional machines has remained relatively unchanged for more than half a century.
However, very recently a totally new machine has been developed for manufacturing bevel and hypoid gears. This new machine is disclosed in PCT application PCT/US87/02083 and U.S. patent application Ser. No. 104,012 filed Aug. 24, 1987, and its operation is remarkably simple in comparison to the conventional machines just referred to above. Namely, while the new machine is capable of all of the complex relative motions produced by conventional machines, these motions are accomplished by moving the new machine's work and tool supports relative to each other under computer numerical control ("CNC") along or about only six axes of movement. The remarkable freedoms provided by this new machine are a definite boon to the bevel and hypoid gear manufacturing industry. Nonetheless, because the complex relative motions needed for generating these gears require that the tool and work supports of the new machine be moved simultaneously along many of the machine's multiple axes, it is important that the accuracy of these fewer axes be monitored regularly, particularly in regard to the manufacture of precision bevel and hypoid gears. Of course, such surveillance by highly trained operators takes time and is part of the expense of the complex manufacturing process described above.
My invention facilitates use of the above-identified new 6-axis machine for the manufacture of precision bevel and hypoid gears, and it is intended to increase and assure the accuracy of the gear products being produced on the new machine and, simultaneously, to reduce the time and expense of this complex manufacturing process.